Yeah, it doesn't work.

I guess it requires a lot of computation that the program is not designed to handle. With JPEG, it has a compression algorithm to play with (and even then it is not very precise). For TIFF, the only thing the program could alter are image dimensions. And to get there, it requires a square root calculation.

To get to, say for the simplicity sake, 24 MP, if an RGB file (i.e., color), it means 8 MP per color (if 8 bits). If a standard full-frame ratio of 1.5, it means the following calculation:

8 divided by 1.5 = 5.333

Square root of 5.33 = 2.309

In other words, you need a file that is 2309 px short and 3464 px long to get to exactly 24 MP.

If you work with a 16 bit file, that would result in a 48 MP (or your calculation would be altered to start with 4 divided by 1.5...)